Cryogenic separation is frequently used to separate substances having low boiling points from one another. Examples of such cryogenic separations include the separation of air into its various components, the production of synthetic gas, and nitrogen rejection from natural gas. A primary limitation of cryogenic separation processes is that they can tolerate, at most, only trace amounts of a freezable component in the feed stream. Concentrations of a freezable component in the feed stream greater than trace amounts would likely result in freezing of the freezable component and plugging of the separation tower.
Diluent gases such as nitrogen and helium are often present in natural gas. The presence of these diluent gases reduces the heating value of the natural gas. Also, certain of these diluent gases may have independent commercial uses if they can be separated from the natural gas. For example, helium is used in the space and superconductor industries. Consequently, the separation of diluent gases from natural gas may have twofold economic benefit, namely, enhancement of the natural gas heating value and production of a marketable gas such as helium. Traditionally, cryogenic processing has been used to separate such diluent gases from natural gas.
One example of a traditional cryogenic process is a nitrogen rejection unit (NRU). Conventional NRU technology requires that CO.sub.2 concentrations in the natural gas feed stream be reduced to 10 to 1,000 parts per million (ppm) to avoid column plugging with solid CO.sub.2. The proposed invention is capable of accommodating significantly higher CO.sub.2 concentrations (i.e., up to about 3 mole %) in the feed stream without column plugging or significant loss of process efficiency.
Two NRU processes which are able to perform adequately over a broad range of nitrogen/methane (N.sub.2 /CH.sub.4) inlet compositions include the single-column heat-pumped cycle and the double-column cycle. Both of these processes are discussed in "Upgrading Natural Gas" by H. L. Vines, Chemical Engineering Progress, November 1986, pp. 47-49.
In a single-column process, the feed stream to the distillation column is precooled with a heat exchanger and then flashed (i.e., ultra fast liquid to vapor conversion) to column pressure. Once the flashed feed stream is introduced to the distillation column, N.sub.2 is removed overhead while CH.sub.4 with some N.sub.2 is condensed in the liquid bottoms stream and reboiled by a heat pump (i.e., a closed-loop methane circulation system). While several hundred ppm CO.sub.2 concentrations in the feed stream may be tolerated, the stream must be maintained at sufficiently high pressure and temperature to avoid solid CO.sub.2 formation. Therefore, much of the work done by the heat pump is used to separate out N.sub.2 at a high pressure. However, if there is no immediate use for high-pressure N.sub.2 the single-column heat-pumped cycle would be less efficient than the double column cycle discussed below.
In a double-column cycle, the N.sub.2 is separated with two sequentially placed columns: a high pressure column followed by a low pressure column. The bulk of the separation is performed in the low pressure column at lower temperatures. Although this process cycle is more energy efficient than the single-column cycle because of this low pressure separation step, it can tolerate only trace levels (i.e., 20 ppm or less) of CO.sub.2 in the feed stream without solids formation. Thus, the single-column cycle offers the advantage of tolerating CO.sub.2 concentrations up to several hundred ppm but at significantly greater energy costs; while the double-column cycle offers the advantage of a more energy efficient N.sub.2 separation but at a substantially lower tolerance for CO.sub.2 in the feed stream.
A double-column cycle taught by Phade et al., U.S. Pat. No. 4,644,686, "Process to Separate Nitrogen and Methane", allows cost effective N.sub.2 separation but requires a complex array of process steps. Additionally, even with Phade's modifications, the double-column cycle cannot tolerate CO.sub.2 concentrations greater than a few hundred ppm.
None of the NRU processes described above can tolerate CO.sub.2 concentrations in the feed stream greater than a few hundred ppm. Often, this necessitates expensive pretreatment steps to reduce the CO.sub.2 concentration to an acceptable level. Accordingly, a need exists for a distillative separation process which can minimize the number of processing steps by increasing the tolerance of higher concentrations of freezable components, such as CO.sub.2, in the feed stream. The present invention satisfies that need.
The present invention extends the scope of the controlled freeze zone technology, disclosed by Valencia et al. in U.S. Pat. No. 4,533,372. As described in that patent, the CFZ process permits separation of a freezable component (e.g., CO.sub.2 or other acid gases), as well as components with lower relative volatility than the freezable component (e.g., butane), from components with higher relative volatility than the freezable component (e.g., CH.sub.4, N.sub.2, etc.).
The present invention demonstrates how significant amounts of a freezable component(s) can be tolerated in a distillation in which the primary separation is between components more volatile than the freezable component(s). A single column NRU is used as an example to show one possible implementation of the invention. However, the invention may also be used in the double-column mode. In either mode the invention demonstrates an orders of magnitude higher tolerance for CO.sub.2 in the feed stream than conventional NRU technology.
It should be noted that relative volatility of the feed stream's components will vary depending upon feed stream composition and the column's temperature and pressure conditions. Consequently, as used herein, "relative volatility" means the comparative volatility of the feed stream components as determined with respect to the feed stream's composition under the temperature and pressure conditions of the distillation column.